1. Field of the Invention
The present invention relates to a magnetic recording device which has a means to record signals as magnetic data optically and reproduce the data with a magnetic head. More specifically, the present invention relates to a magnetic recording device with high resolution and an excellent SN (signal-to-noise) ratio even in high-density recording.
2. Description of the Related Art
With the rapidly increasing amount of digital information spreading in our society in recent years, the storage capacities of storage media, such as magnetic disks, floppy disks, and magnetic tapes and those of optical memory devices, have been increasing quite rapidly. The recording densities of magnetic-disc devices, in particular, have been increasing by 40-60% per annum, and some of them have as high an areal recording density as 4 Gb/in.sup.2. The densification of magnetic recording devices has been supported by such technological innovation as increases in the resolution and the SN ratios of magnetic disks and the sensitivity of magnetic heads. In the case of magnetic disks, their resolution and SN ratios have been enhanced mainly by reducing the size of particles which form their magnetic thin films. However, the approach has a physical limit, i.e., thermal instability. In other words, magnetic particles have to be reduced in size to reduce the noise of the thin films of magnetic recording media, whereas reducing the particle size beyond a certain level causes thermal instability which, in turn, causes the recorded data of the media to vanish. Thus, there is a trade-off relation between the SN ratio and the thermal stability. To solve this problem, the technology of perpendicular magnetic recording and thermomagnetic recording is being developed. The advantage of the perpendicular magnetic recording is that the thin film of a perpendicular-recording disk can be made thicker as compared with that of a conventional longitudinal-recording disk, increasing the volume of magnetic particles. The advantage of the thermomagnetic recording is that the resistance to thermal instability rises because, compared with the magnetic film for magnetic recording, a magnetic film having a very strong coercive force at room temperature is used as the recording film.
In the case of the thermomagnetic recording, data are usually recorded on a disk by radiating a laser beam to the recording layer to reverse the perpendicular magnetization of the recording film. To reproduce the recorded data, the directions of magnetization are read out directly from the recording film or from the copying layer formed on the recording film by utilizing the optical Kerr effect. This optical recording system is capable of forming record bits smaller than the diameter of the beam spot, but has difficulty in reading smaller bits than the beam spot in the process of reproduction, presenting the problem of poor resolution in the range of high linear-recording density. To solve this problem, Japanese Laid-open Patent Publication No. 10-21598 discloses a technique using, instead of an optical head, a magnetic head for magnetic recording/reproducing device s in the process of reproduction. In general, the resolution of a reproducing magnetic head is determined by its shield-gap length of the sensor (gap length), and the gap length of a magneto-resistive head put recently to practical use is reduced to 0.2 .mu.m. The gap length is equivalent to the resolution capable of reproducing 0.1 .mu.m record bits. Since the resolution of current optical disks using a light source of wavelength of 660 nm is about 0.5 .mu.m, the areal recording density can be raised by five times by using the above-described head. In reality, however, that large an enhancement of resolution is difficult to achieve. Its main factor is the shape of the recorded magnetic domains. If data are recorded on a disk with an ordinary optical recording device and recorded optical spots are examined with a polarization microscope, each spot is in a crescent shape as shown in FIG. 4. Its reason is that the sectional shape of the recording beam is circular. On the other hand, the gap portion of a magnetic-reproducing head, which absorbs leakage fluxes from the medium, is in the shape of a rectangle that is long in the direction of width of the track. Therefore, the magnetic-reproducing head cannot efficiently reproduce the magnetic fluxes from the crescent-shaped recorded magnetic domains. The shorter the bit length gets, the lower the efficiency becomes. Thus, even when a magnetic reproducing head is used, sufficiently high resolution cannot be achieved.